The invention relates to a die for ceramic tiles. For the production of ceramic tiles a process which has been in use for some time is one that substantially consists in the pressing of the powders between two half-dies, with the aim of obtaining the compression of the powders and the formation of the tile which will be of a shape determined by the form of the empty space created between the two half-dies when the die is completely closed.
Once the operation has been finished, the tile is sent to be fired, which completes the process. Sometimes, during the firing, some differentiated shrinkage of the tile occurs, leading to a deformation of the tile itself; the tile, according to the entity of the shrinking, is thus considered to be no longer of first quality but of faulty quality, or even waste.
The differentiated shrinking of the parts of the tile is determined by the different density of the parts which are created during the compressing process,
The causes which determine the different density (or rather the non-homogeneity) of the parts of the tile are firstly the non-homogeneity of the powders and, principally, the non-uniform distribution of the powders in the die. If, in a zone of the die, there is a larger quantity of powder, or a powder of higher density with respect to the remaining powder, the tile in that zone will be, after the compressing process, denser with respect to the other parts of the tile and will have a smaller shrinkage if the piece is taken to be fired up to gresification, or it will be less porous in the case in which the piece itself, without shrinkage, remains porous after firing.
To obviate this drawback dies with elastic membranes tensed by underlying pressurised fluid have been used, which technique is well established in the prior art and used for other ceramic products such as plates, insulating elements and the like. The above dies have not, however, up to now, given good results in that the piece that comes out of the dies is deformed by cavities and concavities, which can be acceptable for products already shaped but which is not acceptable for tiles which must have flat laying surfaces both for the subsequent firing and for the laying.
An aim of the present invention is thus that of eliminating the above-mentioned drawbacks by providing a die which permits of obtaining a tile having equal overall density, even in the case where there is non-homogeneity in the powders used or non-uniform distribution of the powders themselves in the die and which enables the tile laying surfaces to be kept flat.
An advantage of the present invention is that of providing a die which is constructionally cheap and usable with normal presses of the types already in use.